Methods and kits for detection of cells in food materials

ABSTRACT

Processes are provided for removal of cells as cell pellets from liquid milk samples, or from cultures or extracts of other food materials or other materials of biological origin. The concentrated cells in the pellet can be analyzed by various techniques to determine the relative cell count, such as by lysing of the cells followed by measurement of ATP. Nucleic amplification, as by the polymerase chain reaction method, can be carried out using the cellular pellet directly, without need for isolation of nucleic acid from the cells. After the amplification, an assay can be carried out for amplified nucleic acid segment indicative of the presence of cells of interest in the sample. The invention thus provides methods for obtaining cellular components from samples of milk, and cultures or extracts of other materials, including food materials, and for determining relative contamination of milk and such other materials by microorganisms. The invention also provides kits for carrying out its various methods.

RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 547,981, filedJul. 2, 1990 now abandoned.

FIELD OF THE INVENTION

The present invention is generally directed to separating andconcentrating cells from non-cellular components in a milk sample, suchas raw milk, pasteurized milk or reconstituted powdered milk, or incultures of microorganisms from other types of food, such as meat, orother sources, such as stool or blood, and then analyzing the separatedcells, as for the presence of undesirable microorganisms.

BACKGROUND OF THE INVENTION

Procedures for counting or quantifying cells in milk samples fall intoone of two categories. In the first category, bovine somatic cells inraw milk samples are counted by various means to identify milk producinganimals that may have bovine mastitis, an undesirable condition whichlimits the quality and quantity of milk production in infected animals.Mastitis testing procedures include direct cell counting using automatedinstruments and bioluminescent somatic cell ATP determinations followingcell lysis with agents such as detergents that release cellularadenosine triphosphate (ATP). The number of somatic cells originallypresent in the sample is estimated from the measurement of the ATPreleased.

In the second category, cells of simple, usually unicellularmicroorganisms, such as fungi and bacteria, referred to hereinaftercollectively as "microorganism cells," are counted in milk samples usingvarious counting procedures. These procedures are generally used in theassessment of milk quality, particularly to screen out grosslycontaminated milk samples.

Of the procedures used for microorganism detection, the Breed Smear(Breed, R. S., Zbl. Bakt. Ilte. Abr. 30:337 (1911)) is generally thequickest method. In this technique, a milk sample is smeared onto aslide, dried, stained, and the bacteria are counted using microscopicexamination. The drawbacks of this procedure are that both viable andnon-viable organisms are counted, and if milk samples contain fewer than10⁵ organisms/ml, many fields in the microscope must be counted toobtain statistically valid results. Microscopic evaluations are tediousand lead to operator fatigue.

The most widely utilized milk microorganism detection method is thestandard plate method, which utilizes direct colony counting afterplating in or on a growth medium. Standard methods (Standard Methods forthe Examination of Dairy Products, 15th Ed., 1985, Richardson, G. H.,Ed., American Public Health Org. Washington, D.C.) have been developedfor milk samples, and many laboratories evaluate milk samples usingeither manual or automated plating procedures. While these methods havebeen utilized worldwide, there are certain disadvantages in using them.First, in the manual plating methods, two or more dilutions of the milksample must be plated so that statistically significant plate numbersmay be obtained. Second, plates for both the manual and automatedplating procedures are usually incubated at elevated temperatures (e.g.,in the U.S., 35° C.; in Japan, 32° C.); and, at these temperatures, thegrowth of psychrophillic organisms is repressed, yielding artificiallylow plate counting numbers. Finally, incubation periods of about 48hours are required before bacterial plates can be counted and stilllonger plating periods are necessary for fungi. During this incubationperiod, bacterial numbers are increasing in the bulk milk from which thesample was taken, so that the result obtained is an underestimate of theactual number of colony forming organisms in the milk after the test.The delay in processing the raw milk to accommodate this incubationperiod by itself lowers the quality of the raw milk, and can contributeto shorter shelf lifetimes of the final product.

Plating or colony counting methods can be either manual or automated.Manual methods include the plate loop method (Thompson, D. I., Journalof Milk and Food Technology 30, 273 (1967)) and the Standard PlateCount, supra.

Semi-automated or automated colony counting methods include the SpiralPlating Method (Gilchrist, J. E., Appl. Micro. 25, 244 (1973)) andmethods carried out by electronic colony counters (Fleming, M. G. Ir. J.Agric. Res. 14, 21 (1975)). The Direct Epi-Fluorescent Technique (DEFT)is a fluorescence-labeling technique which can be performed manually orwith automatic instrumentation. Other techniques include impedancemeasurements after growth of milk organisms and radiometric proceduresutilizing radioactive glucose.

Another approach to microbe evaluation has been the utilization of thebioluminescent measurement of ATP from living cells in milk usingfirefly luciferase (Lumac® bv, The Netherlands). In this scheme somaticbovine cells in raw milk are lysed with a detergent, which releasessomatic cell ATP. This ATP from bovine cells, and any othernon-microbial milk ATP, is degraded with an ATPase, usually potatoapyrase. Finally, a bacterial lysing agent is added and bacterial ATP ismeasured in a bioluminescence assay using firefly luciferase. While muchdata has accumulated in the literature on these methods, the sensitivityhas been inadequate for routine milk testing due to milk and extractantinhibition of the luciferase reaction, incomplete removal ofnon-bacterial ATP, deleterious effects of apyrase on bacterial ATPdetection (Theron, D. N., J. Food Prod. 49:4-7 (1986)) both before andafter bacterial cell lysis, and inefficient extraction of bacterial ATP.Literature data (Webster et al., J. Food Prod. 51:949-954 (1988)) forthis type of assay suggest a cell sensitivity of approximately 1×10⁶cells/ml, which cannot be utilized in the United States where a cutofffor acceptance of 1×10⁵ cells/ml for Grade A raw milk is required.

A logical improvement of this method that has been tried isconcentrating the milk bacteria prior to the ATP assay. Varioustechniques have appeared in the literature which make use of cellfiltration concentration (Peterkin and Sharpe, Appl. Environ.Microbiol., 39:1138-1143 (1980)), although these techniques are quitecumbersome, slow, and still exhibit most of the technical problemsdiscussed above.

Lin et al., J. Dairy Science 72:351-359 (1989), describe a method ofseparating cells from various defined yogurt cultures grown in skimmilk. In the method, 1% EDTA at pH 12 is added to the yogurt culture tobring the EDTA concentration to ≧20 mg/ml of culture and the culture iscentrifuged to separate a cell pellet. The cells of the pellet are thenanalyzed for β-galactosidase activity after being washed with phosphatebuffer and disrupted by sonication. There is no teaching in Lin et al.that the enzymatic activity which they measure is correlated tobacterial contamination of milk. Lin et al. do not teach a method ofseparating cells from, or a method for assessing bacterial contaminationof, milk or any other food or material of biological origin.

While all of the techniques mentioned above have demonstrated somemeasure of success, none has proven to be inexpensive, simple, accurate,fast and sensitive enough to provide the milk industry with a type oftest which can be used satisfactorily for routine testing.

Methods of assaying bacterial cultures grown from food (including milkand meat) samples for Salmonella contamination using an immunoassaytechnique, employing antibodies to an antigen common to Salmonella spp.,and a nucleic acid probe hybridization technique, employing DNA probesfor Salmonella DNA, are available. These methods, however, requirecumbersome and time-consuming growth of bacterial cultures, from thefood material being analyzed, before the assays for Salmonella can becarried out.

The need for such culturing prior to application of nucleic acid probehybridization techniques to detect contamination, by undesirablemicroorganisms, of food materials, including milk, meat (e.g., chicken,turkey, beef, pork, horse, goat, whale and the like), eggs, fish,mussels, molluscs, crustaceans, vegetables, fruits, grains, and the likecan be avoided, or the time for culture significantly reduced, ifnucleic acid amplification techniques, such as the well known polymerasechain reaction (PCR) technique, are employed to increase theconcentration of nucleic acid segments, characteristic of microorganismcontaminants, to levels that are readily detectable by nucleic acidprobe hybridization or nucleic acid staining methods. However, targetnucleic acid amplification techniques have not been successfully appliedfor this purpose with cultures or extracts of specimens of foodmaterials, or other materials of biological origin, such as blood, urineor stool, unless, prior to application of processes for amplification,microorganisms from the specimens have been subjected to cumbersome,costly, and otherwise undesirable, special treatments, such as withproteolytic enzymes, high concentrations of guanidinium salts, anddetergent, or boiling, and the DNA from the microorganisms has beenprocessed by similarly undesirable procedures such asethanol-precipitation or chromatographic separation with or withoutphenol/chloroform extraction. These treatments have been regarded asnecessary to separate nucleic acid, to be subjected to amplification,from contaminants, that interfere with the enzymatic reactions necessaryfor the amplification and that are thought to be provided by themicroorganisms themselves or otherwise provided from the specimens tothe microorganisms separated therefrom. See, e.g., Hill et al., Appl.Environ. Microbiol. 57:707-711 (1991); Keasler and Hill, Abstracts ofthe 91st General Meeting of the American Society for Microbiology,Abstract No. P-12, p. 269 (1991); Olive, J. Clin. Microbiol. 27:261-265(1989).

SUMMARY OF THE INVENTION

The present invention provides a method for the concentration of cells,either mammalian or microorganism, from a liquid milk sample, which canbe carried out in a simple and reliable manner. The present inventioncan be used to analyze various types of liquid milk products, includingraw milk, pasteurized milk, reconstituted dry milk, cream, ice cream andother milk products and derivatives.

The method of the present invention comprises adding a chelating agent,usually in a solution referred to as a "clearing solution," to a milksample, and then separating the cellular components from other milkcomponents, such as by centrifuging the sample combined with chelatingagent for a brief period of time, and separating or decantingnon-cellular milk components from the separated cellular pellet. In apreferred separation method, involving centrifugation, microparticulatecarrier, such as small polystyrene beads, that sediments slightly moreslowly than the cells, is included in the sample during thecentrifugation. An agent, such as a detergent, to lyse animal cells butnot microorganism cells of interest, may also be included.

The components of the cellular pellet resulting from the separation areamenable to a variety of analyses, with such analyses being free frominterferences that would be caused by contaminating milk components. Forexample, the cells of the pellet can be isolated in a manner that makesthem amenable to quantitation using ATP bioluminescence techniques.

The cellular pellet may contain both somatic cells (i.e., mammaliancells from the mammal that is the source of the milk) and microorganismcells. Cells from the pellet can be microscopically examined todetermine the relative concentration of somatic and microbial (i.e.,microorganism) cells. The somatic cells may also be removed by adding alysing agent, such as a detergent, to the sample with the chelatingagent in the clearing solution, with an agent being chosen which lysesonly the somatic cells. The pellet remaining after centrifugation willthus contain almost exclusively microbial cells, which can be analyzedin various ways, including analysis for ATP concentration afterextracting ATP such as by lysing of the microorganisms. Filtration mayalso be utilized to separate cellular components from other milkcomponents in the milk sample to which the clearing solution has beenadded.

The presence of animal cells, such as bovine somatic cells, in a rawmilk sample, can also be determined in a quick and simple manner. Theinvention is further suited to the analysis of contamination of milksamples by a variety of microbes (microorganisms) such as bacteria,yeasts and molds, and spores from bacteria, yeasts and molds.

The present invention is also directed towards the utilization ofconcentrated cellular materials isolated from milk, or other foods ormaterials of biological origin, for various types of further analysesfor undesirable contamination. Such analyses include those utilizingnucleic acid amplification by any of various methods, includingpolymerase chain reaction (PCR) methods, and those employing microscopicexamination following use of dyes or stains, or other chromogenic,fluorescent, chemiluminescent or other detectable reporter molecules, torender observable cells to be detected. The invention is also directedtowards the use of the concentrated cellular material for either broadspectrum or specific plating of microorganisms, or for preparation ofsamples in other forms, for any of the methods that have been utilizedin the art for assessing bacterial or other microbial contamination ofmilk, other food stuffs, or other materials of biological origin.

The beetle luciferase ATP determination method (DeLuca, M. A., Advancesin Enzymology, Meister, A., editor, 44, 37-68 (1976)) may be utilized inthe present invention for quantification of cellular numbers in theoriginal liquid milk sample. See, also, International ApplicationPublication No. WO 92/04468. This ATP determination method is applicableto the quantification of either animal or microorganism cell numbers.Other methods that may be employed in accordance with the presentinvention for quantification of cell numbers in a liquid milk sample, orsample of other food or material of biological origin, are nucleic acidprobe hybridization assay methods, including such assays followingtarget nucleic acid amplification by PCR or other amplificationtechniques.

The method of the present invention may also be used as a pretreatmentmethod for a variety of samples for centrifugation or filtration onvarious types of filtration matrices such as membranes, hollow fibers orcentrifugation type filters.

While chelating agent, detergent (lysing agent), and microparticulatecarrier each contribute important, independent advantages in theseparation of cells from milk samples, in the case of other food samplesthe combination of microparticulate carrier with either centrifugationor filtration is sufficient.

Surprisingly, microorganism cells from milk samples, or from cultures orextracts of prepared from samples of other food materials, as indicatedabove, or non-food materials of biological origin, such as blood, urineand stool, after separation from non-cellular components using achelating agent, microparticulate carrier/clearing solution (in the caseof milk) or a microparticulate carrier/clearing solution (in the case ofother materials) and separation method in accordance with the invention,are free of contaminants, other than those present in the separatedcells themselves, that inhibit enzymatic reactions that are necessaryfor target nucleic acid amplification. Thus, advantageously, theseparated cells can be used directly, without need for isolation ofnucleic acid from other cellular constituents, to provide nucleic acidfor amplification. The cells simply need to be treated (as by being heldbriefly at an elevated temperature) to cause them to release nucleicacid, and to denature enzymes or other components that would degradeamplified nucleic acid or inhibit enzymatic reactions required for theamplification method employed. If the amplification method relies on anenzyme that would be irreversibly inactivated at the elevatedtemperature, the cells can be added directly to a solution of reagentsrequired for amplification, other than the temperature-sensitiveenzyme(s), the solution can be heated to the elevated temperature andthen cooled, and the enzymes can be added to initiate amplification(assuming target nucleic acid is present). In a preferred method, onlythermally stable enzymes will be used in the amplification process(preferably PCR). Then separated cells are added directly to a solutionwith all reagents, including enzymes, required for amplification, andthe solution is heated to an elevated temperature (low enough so thatthe enzymes for amplification remain active), and then, when thetemperature is reduced sufficiently for primers to hybridize totemplate, amplification begins. Target nucleic acid segment from themicroorganisms sought to be detected, if present, will be amplified inthe amplification process. The nucleic acid resulting from theamplification process can then be assayed, using, for example, anynucleic acid probe hybridization assay method, for target nucleic acidsegment.

Thus, the invention provides a method for testing milk products, otherfood materials, and other materials of biological origin, such as animalblood, urine or stool, for contamination by preselected microorganisms,characterized by having a particular target nucleic acid segment,comprising separating microorganism cells from the milk or othermaterial of biological origin, or from a culture prepared with saidmaterial, using the cell separation methods of the invention (sometimesreferred to as the cell wash methods); suspending microorganism cells soseparated in a solution and treating the solution to provide nucleicacid from the cells for amplification without substantial isolation ofnucleic acids of the cells from other constituents of the cells,subjecting the nucleic acid of the resulting solution to a nucleic acidamplification process, whereby target nucleic acid segment, if present,will be amplified or will cause the amplification of another,predetermined segment, and assaying the nucleic acid after theamplification process for the presence of amplified segment. The cellsof the pellet yielded by the cell wash methods may be simply washedprior to being suspended in the solution for treatment to providenucleic acid for amplification. Preferably the assay, which is the finalstep of the method, will be a nucleic acid probe hybridization assay,which, as the skilled will understand, will employ a probe (preferablyan oligonucleotide and labelled to be detectable) capable of hybridizingto amplified target segment. As the skilled will understand, by carryingout the method in parallel with appropriate standards, the method can beused to not only detect the presence of preselected, contaminatingmicroorganisms (if present at a level above the sensitivity of themethod) in the material being tested with the method, but also quantifythe extent of contamination of the material with such microorganisms. Anelevated temperature, which can be used to release nucleic acid fromcells and denature or inactivate amplification-inhibiting substancesfrom the cells, is above about 70° C., and more preferably above about90° C., for analysis of samples of cells from milk, or cultures preparedfrom meat, eggs, or aquatic species used as food sources.

In preferred amplification processes, aliquots of solution with reagents(e.g., enzyme) will not need to be added over the course of theamplification process. One preferred amplification method is the PCRmethod using a thermally stable DNA polymerase from a bacterium, such asThermus aquaticus, which lives at high temperatures; in this method,thermal cycling will occur but the thermally stable polymerase willretain sufficient activity, notwithstanding the high temperaturesreached in the cycling, that enzyme will not need to be replenishedafter each thermal cycle during the amplification process.

The present invention also encompasses novel kits for use in carryingout the methods of the invention. Such kits include a clearing solution,which, in the case of kits for milk samples, contains at least chelatingagent and optionally microparticulate carrier or a somatic cell lysingagent, and, in the case of kits for other materials of biologicalorigin, contains at least a microparticulate carrier. The kits also maycontain a solution for washing or treating the cell pellet to lower thelevel of residual ATP background and, for ATP detection, a microbialcell ATP extracting agent or lysing agent, optionally a buffer solution,and an ATP detection reagent such as luciferase-luciferin. Kits may alsoinclude components for preparing a sample for the Breed Smear test orother types of direct microbiological examination procedures. Kits ofthe invention may also comprise enzymes, buffers, and other reagents fornucleic acid amplification or detection of amplified nucleic acid in anucleic acid probe hybridization assay or by staining.

Further objects, features and advantages of the present invention willbe apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 illustrates the steps in the general method of the presentinvention using a liquid milk sample.

FIG. 2 is a graph showing plots of relative light units (RLU) in samplesanalyzed in accordance with the present invention and by anotherprocedure versus the concentration of microorganisms in the samples asdescribed in Example 1.

FIG. 3 is a graph showing the correlation between colony forming units(CFU) and RLU for the separation of microbial cells from milk describedin Example 2.

FIG. 4 is a graph showing the correlation between CFU and RLU for theseparation of microbial cells from milk described in Example 3.

FIG. 5 is a graph showing the correlation between CFU and RLU for theseparation of microbial cells from milk described in Example 4.

FIG. 6 is a graph showing the correlation between CFU and RLU for theseparation of microbial cells from milk described in Example 5.

FIG. 7 is a graph showing the correlation between RLU and the amount ofartificially inoculated milk in samples tested as described in Example6.

FIG. 8 is a graph showing the correlation between CFU and RLU in theseparation of microbial cells from milk as described in Example 9.

FIG. 9 is a graph showing the correlation of RLU to CFU when differentchelators or chelating conditions are employed as described in Example10.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present invention is directed to a method for theseparation and concentration of cellular materials from liquid milksamples or from cultures prepared from other food materials or fromother materials, such as blood or stool, of biological origin. Cells areremoved from milk samples or cultures by adding a clearing solutionincluding a chelating agent, and optionally a microparticulate carriersuch as polystyrene beads or an animal cell lysing agent such as adetergent, to the milk sample, and separating the cellular componentsfrom non-cellular components, such as by centrifuging the sample, andaspirating or decanting the non-cellular supernatant from the cellularpellet. The cells in the pellet can then be analyzed for, for example,microbial contaminants of the milk or other material.

The invention also encompasses a method for detecting the presence ofcells, characterized by having nucleic acid that comprises a preselectedtarget segment, in a culture or extract of a material of biologicalorigin, said method comprising obtaining a pellet of cells from theculture or extract by combining an aliquot of the culture or extractwith an aqueous suspension of a microparticulate carrier to form aclearing solution and centrifuging the clearing solution to form a cellpellet, provided that, if said culture of a material of biologicalorigin is a liquid milk sample, said clearing solution further comprisesa chelating agent; suspending cells from the pellet so obtained in afirst solution and treating the first solution to provide a secondsolution of nucleic acid from the cells for amplification withoutsubstantial isolation of nucleic acids of the cells from otherconstituents of the cells; subjecting the nucleic acid of the secondsolution to a nucleic acid amplification process to provide apre-determined, amplified nucleic acid segment only if the pre-selectedtarget segment is present in said cells suspended from said pellet; andassaying nucleic acid after the amplification process for the presenceof said pre-determined, amplified segment.

The invention also provides kits to carry out the methods of theinvention.

The methods of the invention are preferably carried out with liquid milksamples.

Definitions:

The term "bacteria" is meant to include single-celled prokaryotes. It isalso within the scope of the present invention to interchange the terms"microbe" or "microorganism cell".

The term "eukaryotic cells" is intended to denote organisms in which thegenetic material is enclosed by a nucleus.

The term "liquid milk sample" is meant to include all liquid solutionsof origin from dairy raw materials or products including raw milk, ultrahigh temperature pasteurized milk, low temperature long time pasteurizedmilk, reconstituted powdered milk, cream, skim milk, liquefied ice creamor ice milk or related products, and suspensions of milk or dairyproducts in liquid samples. While bovine milk is the preferred materialfor application of the present invention, the invention is applicable aswell to milk from any mammal.

The term "chelator" or "chelating agent" is meant to include allmolecules or macromolecules that bind to or combine with calcium ionsand may also bind with other divalent metal ions including magnesiumion, iron ion, zinc ion, cadmium ion, beryllium ion, cobalt ion, nickelion, copper ion, lead ion, and other metal ions. The term "chelator" or"chelating agent" includes all synthetic and natural organic compoundsknown to bind these ions, and any molecule of biological origin, orby-product or modified product of a molecule of biological origin, suchas proteins, sugars or carbohydrates, lipids and nucleic acids, and anycombination thereof, that may bind the above mentioned types of ions.The term "chelator" or "chelating agents" also includes any solidmaterial of naturally occurring or synthetic origin that binds calciumand to a lesser extent magnesium and perhaps one or more of the otherabove-mentioned ions.

While various procedures utilized in the overall invention are generallyknown to the art, the combination of these procedures in accordance withthe invention has not been contemplated. General methods known to theart, which play a part of the overall assay techniques of the presentinvention, include the standard plating techniques of microorganismsfrom dairy products, staining and identification methods formicroorganisms, bacterial extraction methods, the use of separated,concentrated cellular materials in other procedures, and generalchelating chemistry. A discussion of one or more of the above-notedtechniques can be found in the following references, which areincorporated herein by reference: Standard Methods for the Examinationof Dairy Products, 15th Ed., 1985, Richardson, G. H., Ed.; AmericanPublic Health Assoc., Washington, D.C.; Bacteriological AnalyticalManual, 6th Ed., United States Department of Agriculture (USDA), 1984,Marcel Dekker Inc., New York; Sambrook J., Fritsch, E. F., and Maniatis,T., Molecular Cloning, 2nd Ed., Ferguson, M., Ed., Cold Spring HarborLaboratory Press, 1989; O'Connor, F., Australian J. Dairy Tech., pp.61-65 (June, 1984) (this reference includes descriptions ofmicrobiological milk testing methods and relevant references for eachmethod); Karl, Microbiological Reviews 44, 739-769 (1980); Martell, A.E., Chemistry of the Metal Chelate Compounds, Prentice-Hall, New York,1952; Current Protocols in Molecular Biology, John Wiley & Sons, NewYork, N.Y., 2 volumes (supplemented)(1987-1992).

As indicated in the cited Molecular Cloning and Current Protocols inMolecular Biology, nucleic acid amplification techniques, particularlythose with thermally stable enzymes, such as PCR amplification employingDNA polymerases from Thermus aquaticus, are well known, as are nucleicacid probe hybridization assay methods and staining methods fordetecting nucleic acid segments. Additional information on suchtechniques can be found in U.S. Pat. Nos. 4,693,195 and 4,693,202 (PCRand its application in nucleic acid probe hybridization assays fordiagnosis); International Patent Application Publication No. WO 88/10315(transcription-based nucleic acid amplification/detection methods); U.S.Pat. No. 4,889,818 and International Patent application No.PCT\US90\04169 (published February, 1991)(thermostable DNA polymerasesfrom Thermus aquaticus (Taq DNA polymerases) and their use in PCRamplification); European Patent Application Publication No. 0 329 822(isothermal transcription-based nucleic acid amplification process);U.S. Pat. No. 4,957,858 (Q-Beta replicase catalyzed autocatalyticreplication of replicatable RNA linked to probes complementary to targetsegment).

The present invention encompasses a separation and concentrationtechnique for the removal of cellular materials from liquid milk samplesand other cultures. While the technique will now be described withreference to a liquid milk sample, it will be understood that it may beapplied as well to cultures prepared from other materials.

In carrying out the technique, a milk sample is placed in anappropriately sized centrifugation vessel as shown at 10 in FIG. 1 and achelating agent, and optionally a microparticulate carrier or a lysingagent, are added. The chelating agent may be one of various types asdescribed above, which by way of illustration only, may includeethylenediamine tetraacetic acid (EDTA, Versene®),bis-(o-aminophenoxy)-ethane-N,N,N¹,N¹ -tetraacetic acid (BAPTA),ethyleneglycol-bis-(β-aminoethyl ether) N,N,N¹, N¹ -tetraacetic acid(EGTA), nitrilotriacetic acid (triglycine, ammonia triacetate, Trilon Aor, herein, simply "Trilon"), trans-1,2-diaminocyclohexanetetraaceticacid (CDTA), diethylenetriaminopentaacetic acid (DTPA), N-(2-acetamido)iminodiacetic acid (ADA), citrate, arginine, hypoxanthine,4,5-dihydroxybenzene-1,3-disulfonic acid, sodium phosphate glass or anyof the molecules in the "glass" or polyphosphate family, crown ethertype compounds and all derivatives and precursors of such molecules.

Among the chelators that may be employed are nitrilotriacetic acid andderivatives thereof, of Formula I

    (HOOC(CH.sub.2)).sub.2 NR.sub.1                            I

wherein R₁ is selected from the group consisting of --(CH₂)COOH;--(CH₂)CONH₂ ; --(CH₂)CO(CH₂ OH); --(CH₂)(CO)COOH; --(CH₂)(CO)COCH₃ ;--(CH₂)(CO)(CH₂)COOH; --(CH₂)(CO)(OCH₂ CH₂)_(q) OH, wherein q is aninteger from 1 to 30; ##STR1## wherein R₂ is selected from the groupconsisting of --COOH, --NH₂, and --N((CH₂)COOH)₂ ; --(CH₂)(S(O)_(r))CH₃,wherein r is 1 or 2; --(CH₂)SO₃ H; --(CH₂)(S(O)_(r))(CH₂)COOH, wherein ris 1 or 2; --(CO)CH₂ COOH; --(CO)COOH; ##STR2## --(CH₂)(CO)OR₃, whereinR₃ is selected from the group consisting of phenyl substituted at anyone position with hydroxyl, ##STR3## and ##STR4## wherein R₄ is selectedfrom the group consisting of H, --COOH, --CONH₂, --OH, and --SO₂ H;##STR5## wherein q is an integer of 1 to 4 (i.e., the pyridine ring issubstituted at 1 to 4 positions with --(COOH) ); --(CH₂)(CO)(CH_(n)(X₁)_(m)) , wherein m and n are both integers from 0 to 3, m+n=3, andX_(x) is selected from the group consisting of --Cl, --Br, --F, --CN and--NO₂ ; --(CH₂)(CHOH)_(p) (CH₂ OH) , wherein p is an integer from 0 to9; --(CH₂)(CO)(C(X₂)₂)(CO)C(X₂)₃, wherein X₂ is selected from the groupconsisting of --H, --Cl, --Br, and --F; and --(NH)(CO)(CH₂)COOH. As theskilled will understand, various salts (e.g., sodium salts) and anionicforms of the chelating agents described herein as acids will be employedin carrying out the invention.

The chelating agent, for example EDTA, Trilon or ADA, and carrier orlysing agent, if present, are preferably added to the milk sampletogether in a solution, called a "clearing solution"; and the mixture ofchelating agent, and carrier or lysing agent, and milk sample is cappedand inverted or vortexed to mix. The sample is then placed into acentrifuge of appropriate size, and the sample centrifuged at 10,000× g(minimum relative centrifugal force) for a minimum of 5 minutes. Aftercentrifugation the sample separates into three distinct phases. Theuppermost phase is a cream and milk protein "pad" illustrated at 15 inFIG. 1, and this pad floats at the very top of the liquid sample.Beneath the pad is the second "clear" liquid zone, illustrated at 16 inFIG. 1, which is, unlike a milk sample, non-opaque and translucent.Finally, at the bottom of the centrifuge tube is the cellular pellet 17,the size of which is dependent on the number of cells in the originalmilk sample and the type of chelating agent(s) and/or microparticulatecarrier and/or detergent(s) used in the treatment. The pellet may alsocontain a small amount of other milk components which are associatedwith the cells. After clearing of a typical 1.0 ml raw milk sample, thepellet is approximately 10 to 40 μl in volume and is white or off-whitein appearance.

The proposed effect of the chelating agent in the present invention isthe dissociation of casein micelles in a milk sample into "sub-micelles"(Chaplin, J. Dairy Res. 51:251-257 (1984)). After the chelatingtreatment, micellular milk protein material rises to the top of acentrifuge tube or remains in solution, as opposed to pelleting in theabsence of chelator. The chelator binds calcium ion which is a majorcomponent that contributes to micelle structure (Lia, Biochemistry11:1818-1821 (1972)). Thus, chelating agents that bind calcium ionparticularly well are preferred in the present invention.

The ideal chelator, for the isolation of cells to be quantitated usingmeasurement of ATP by bioluminescence, must fulfill two requirements.The first requirement is the separating capability mentioned above toreduce nonmicrobial ATP background to acceptable levels. The secondrequirement which must also be met is the maintenance of microbial ATPpools for subsequent determination and quantification.Chelating/centrifugation procedures previously described (Lin, et al.(1989), supra) do not meet this second requirement because EDTAchelating at high pH levels (pH12) significantly reduce bacterial ATPlevels. A chelating agent that can simultaneously reduce milk ATPbackground, while maintaining microbial ATP pools is highly preferred inthe present invention and particularly in applications of the inventionwhere ATP is to be measured to assess microbial concentration orcontamination. In this regard, chelating agents which have been found tobe particularly suitable are nitrilotriacetic acid and N-(2-acetamido)iminodiacetic acid.

However, ethylenediamine tetraacetic acid is preferred as a chelatingagent in applications of the invention where nucleic acid amplificationis carried out with nucleic acid from the separated cell pellet and thenmicrobial concentration or contamination is assessed by a nucleic acidprobe hybridization assay with the amplified nucleic acid.

The present invention may also employ microparticulate carrier for thecollection of cells, especially microbial cells, during thecentrifugation steps of the cell separation process. Themicroparticulate carrier serves to assist in the pelleting process. Thephysical nature of the microparticulate carrier is such that the carriersediments, or pellets, slightly slower than the microbial cells and assuch makes the cell collection more quantitative.

A number of materials could be used as microparticulate carriers,including "beads" of polystyrene, latex, plastic, glass, diatomaceousearth, metal oxides, and colloidal materials including polyacrylamide,dextrans, cross-linked dextrans, and starches. The properties of themicroparticulate carrier that are desirable are threefold: 1) Thecarrier should sediment as fast as, or preferably slightly slower than,the cells of interest that are being collected. This characteristic isbasically a function of the particles' density, size and charge. 2) Thecarrier should be amenable to resuspension after centrifugation for thepurpose of facilitating treating or evaluating the cell pellet. 3) Thecarrier must be inert to the testing or evaluation that will beperformed on the cell pellet. For example, the carrier should beinvisible in cell-staining techniques if a microscopic examination is tobe performed, or the carrier should not bind an analyte, such as ATP,that will be measured to measure or estimate numbers or concentrationsof microbial cells. Beads of materials cited above with diametersbetween about 0.25 μm and 2.5 μm are suitable as microparticulatecarriers. Surfactant-free polystyrene beads with a diameter from 0.5 to1.5 μm are the preferred microparticle carriers.

The use of microcarrier particles in the cell separation method of theinvention enhances the ease with which an operator is able to removeunwanted supernatants from cell pellets following a centrifugation. Thecarrier serves as a visual indicator much larger than the microbial cellpellet and as such makes the removal of the supernatants much moreconvenient. In addition, the probability of mistakenly aspirating partor all of the cell pellet is greatly reduced when a microparticlecarrier is used.

If one wishes to remove somatic cells from a raw milk sample andseparate out and concentrate only microbial cells, a somatic cell lysingdetergent, such as non-ionic detergents Triton X-100, Nonidet P-40(NP-40), or the like, may be added to the milk sample in combinationwith the chelating agent. Such a detergent will specifically lyse bovinesomatic cells (animal cells) without lysing microbial cells. Because thesomatic cells are lysed in the treatment prior to centrifugation, therewill be essentially no intact somatic cells in the post-centrifugationpellet. Subsequent ATP determinations made on these pellets will thusdetect only the bacterial cells or other microbe cells.

If, on the other hand, one wishes to measure or quantify the number ofsomatic cells in a milk sample, a somatic-cell lysing detergent, such asone of those mentioned above, can be added to the cellular pellet andonly somatic cells will be lysed. A cellular metabolite such as ATPwhich was contained in the somatic cells can then easily be measuredwithout any elevation of the result by microbial ATP, which cannot beextracted with the somatic-cell-lysing detergent. The number of somaticcells that were present in the sample can then be calculated using theknown amount of ATP that was measured and the average amount of ATPknown to exist in somatic cells. This procedure is an improvement overpreviously published methods (R. Bossuyt, Milchwissenschaft 33:11-13(1978)).

The present method provides a useful procedure for concentrating cellsand eliminating background milk contaminants such as casein and caseinmicelles, lipophilic components, and salts. To perform thisconcentration, milk, for example one ml, is cleared by centrifugationwith a chelating agent, and the resulting pellet is resuspended in avery small volume (for example, 10 μl) of the appropriate buffer orliquid. In this example, all of the cell components are removed from themilk contaminants and are concentrated 100-fold. This concentratedsample can then be utilized in other analyses as desired.

One example of the usefulness of this procedure is in staining andcounting microorganisms from raw milk. If a milk sample contains lessthan 1×10⁵ organisms per ml, and 10 μl of milk is put onto a microscopeslide and stained, many fields of the stained specimen must be viewed inorder that statistically significant counting data be collected. Thefields are difficult to score because of background staining of othermilk components. This procedure, the Breed Smear procedure, is widelyused, particularly in Japan, for raw milk quality assessment. Byutilizing the foregoing concentration step (100 fold concentration) manyfewer fields need to be counted and fields are much easier to scorebecause the background is much clearer. This speeds up specimenthroughput and reduces operator fatigue and error.

Somatic cell numbers in a specimen may be quantitated by separating allcells from the various other components of milk and concentrating thecells in accordance with the present invention.

Microbial cells in a pellet, which is free from contaminating somaticcells by virtue of use of a somatic-cell-lysing detergent as outlinedabove, can be lysed with a microbial extracting agent and assayed forATP. The number of microbial cells is then estimated using the ATPmeasurement and the average amount of ATP known to exist in microbialcells or, more specifically, in milk microbial cells. This type ofprocedure offers a number of advantages over other methods such asstandard plating and spiral plating. First, it is much faster, because aresult is available in as little as 15 minutes, and consistently inunder an hour, as opposed to the 48 hour time frame for the standardplating and spiral plating methods. Second, the ATP measurement willgive results that are not influenced by cell clumping, i.e., ATP will bemeasured from all cells; in plating methods cell pairs or groups ofcells are scored as a single cell because one colony is formed. Third,the ATP method will measure ATP from psychrophillic organisms which willbe missed in plate counting methods that culture at temperatures over30° C.

As mentioned before, cells which are separated and concentrated usingthe present invention can be subjected to various other methodologies(e.g., treatment with a protease, such as α-chymotrypsin, and/or anATPase such as an apyrase) with the background greatly reduced and thecell number (and associated signal) greatly increased due toconcentration. This provides the possibility of greater sensitivity, andmore reliable, reproducible results due to background elimination.

A method of the present invention may also include cell enrichment orenhancement techniques either before or after the clearingcentrifugation so that sensitivity, cell number, or cell hardiness maybe improved. For example, by treating cells with treatments (Theron, J.Food Prot. 46:196-198 (1983)) or components (K. M. Oxley,Bioluminescence and Chemiluminescence, Ed. J. Scholmerich, John Wiley &Sons, N.Y., pp. 495-198 (1986)) that increase cellular ATP, thesensitivity of the procedure may be enhanced. This type of procedure canalso be applied to detecting specific types of cells in a particularsample, using selective media or even polyclonal or monoclonalantibodies.

Persons of skill in the art are well aware of the need for, and natureof, appropriate controls in assay procedures of the type employed in thepresent invention and appropriate standards to allow quantitativeinformation on analyte concentration or quantity to be determined fromsuch assay procedures. Such controls and standards are illustrated inthe following Examples.

Reference is now made to FIG. 1, where the clearing wash method of thepresent invention is illustrated with reference to a milk sample. Inthis procedure, a milk sample is first placed into a centrifugationvessel 10. Next, a chelating agent, plus (preferably) or minusmicroparticle carrier, and plus or minus a somatic cell lysingdetergent, is added using a clearing solution, as indicated. Thecontents of the vessel are mixed, the vessel is then centrifuged, andthe resulting sample is said to be "cleared". If a milk sample without achelating agent were to be centrifuged in the same manner, there wouldbe no "clearing" but a large, diffuse, milk protein and cell pellet atthe bottom of the tube. Finally, the cell pellet is freed from most ofthe other materials in the vessel by aspiration of the supernatant afterthe centrifugation. The addition of the microparticle carrier greatlyassists in the collection of microbial cells, and in the aspiration andremoval of non-microbial milk components.

Kits for the analysis of cellular components from liquid milk samplescan be formed of the following components:

A microbial test kit using ATP detection comprises:

(a) A clearing solution containing a chelating agent, a somatic celllysis detergent such as the chelating agents and detergents describedabove, and, optionally and preferably, a microparticulate carrier.

(b) Optionally, a solution for washing or treating the cell pellet.

(c) An ATP extractant such as a cell lysis solution, as described above,for releasing microbial cell ATP.

(d) Optionally, a buffer solution.

(e) An ATP detection reagent, such as luciferase (e.g., from P.pyralis)-luciferin for measurement of ATP by bioluminescence.

A microbial test kit using the Breed Smear comprises:

(a) A clearing solution as described for the microbial test kit usingdetection of ATP.

(b) Optionally, a solution for washing the cell pellet.

(c) A solution for staining cells.

A bovine somatic cell test kit using ATP detection comprises:

(a) A clearing solution as described above for the microbial test kitusing detection of ATP, excluding the somatic cell lysis detergent.

(b) A somatic cell lysis solution that does not lyse microbial cells.

(c) An ATP detection reagent such as luciferase-luciferin.

A microbial detection kit for detection of microorganisms via nucleicacid amplification and nucleic acid probe hybridization comprises:

(a) A clearing solution which comprises a microparticulate carrier and,in the case of a milk sample, will also comprise a chelating agent.

(b) Optionally, solution for washing the cell pellet from the cell washmethod using the clearing solution.

(c) Enzymes and primers required for amplification of a target nucleicacid segment characteristic of the microorganisms to be detected.Optionally, other, more commonly available components used in theamplification, such as nucleoside triphosphates (2'-deoxyribonucleosidetriphosphates in case only DNA is synthesized in the amplificationprocess), buffers, and the like may be provided.

(d) Nucleic acid probe for target segment amplified with the componentsin (c) together with reagents required to detect probe hybridized tosaid target segment, if the probe per se is not detectable (e.g., aslabelled with ³² p). For example, if the probe is labelled covalently(i.e., directly) with an enzyme, such as alkaline phosphatase, ornon-covalently (i.e., indirectly), through biotin covalently linked tothe probe, with an avidin-enzyme (e.g., alkaline phosphatase) complex,wherein the enzyme catalyzes production of a chromophore, which providesthe detected signal, then reagent for production of the chromophore and,for indirect labeling, avidin-enzyme complex will be provided. Again,buffers, solid supports for hybridization, and the like may also beprovided.

(e) Alternatively, or optionally in addition, to nucleic acid probe andassociated reagents as in (d), a staining reagent such as ethidiumbromide to detect amplified target of known size, as on a gel. Reagentsfor preparing a suitable gel, running in parallel a sizing gel, and thelike may also be provided, as the skilled will understand.

Each of the foregoing kits may optionally include a filter, for example,as described in Example 6 below, and reagents to provide suitablecontrols or, for quantification, standards.

The invention is further illustrated by the following, non-limitingexamples.

EXAMPLE 1

This example discloses the separation of microbial cells from anartificially inoculated milk sample. In addition, the assay is comparedto a commercially available milk ATP assay to compare the relativesensitivities of the two procedures.

Raw milk samples were obtained from a local dairy and the raw milk wasstreaked on standard methods agar (Standard Methods for the Examinationof Dairy Products, supra) to isolate individual colonies. Elevenvisually different colony types were picked and grown in standardmethods broth in an attempt to obtain a representative sample of thedifferent species that may be found in raw milk. Of these isolates onecell type (Serratia liquefaciens) was chosen for the experiment.

A pasteurized milk sample was used as a negative control for theprocedure. To one ml of this pasteurized milk was added 10 μl of theovernight grown (23° C.), milk-isolated bacteria (S. liquefaciens).Using this artificially inoculated milk sample and uninoculatedpasteurized milk, a number of inoculated milk samples were prepared asset forth in Table 1 below.

                  TABLE 1                                                         ______________________________________                                                     Pasteurized                                                                             Inoculated                                             Tube         Milk (μl)                                                                            Milk (μl)                                           ______________________________________                                        1            1000       0                                                     2            999        1                                                     3            998        2                                                     4            995        5                                                     5            990       10                                                     6            980       20                                                     7            950       50                                                     8            900       100                                                    9            800       200                                                    10           500       500                                                    11            0        1000                                                   ______________________________________                                    

To calculate the number of organisms put into the inoculated milksample, 100 μl of serial dilutions (10⁻³, 10⁻⁴, 10⁻⁵, 10⁻⁶, 10⁻⁷, and10⁻⁸) at the overnight cell suspension were plated on standard methodsagar and incubated overnight at 37° C. The resulting colony countingdata was used to calculate the number of bacteria in tubes 2-11.

A series of eight 1.5 ml micro-centrifuge tubes were placed into a testtube rack and numbered, and 1.0 ml of each of the samples 1-8 in Table 1was placed into each tube. The milk was incubated at room temperature(22° C.) for 10 minutes, and 500 μl of a 0.25M EDTA, pH 8.0, 0.5%Nonidet P-40 (NP-40) solution was added to each tube. The tubes werecapped, inverted 10 times to mix, and placed into the rotor of an anglehead microcentrifuge (Wheaton) being careful to have the tube hingespointing outward. The tubes were centrifuged at 12,000× g for 10 minutesand the supernatant was then carefully removed using a 1000 μldisposable pipet tip which was attached to a faucet aspirator. The tubewas tipped slightly to pour the last remaining supernatant into theaspirator, so as not to aspirate, or otherwise disrupt, the resultingpellet.

To each pellet 500 μl of a 0.1M MgCl₂, 0.2% NP-40 solution was added,the tubes were capped and vortexed to resuspend the cells in the pellet,and then the tubes were centrifuged as before. After centrifugation, thesupernatants were again aspirated and another 500 μl of 0.1 M MgCl₂,0.2% NP-40 was added to each tube. The tubes were capped, centrifugedand aspirated as before.

To each tube was added 50 μl of a 1% trichloracetic acid (TCA), 1 mMEDTA, 0.0005% xylenol blue solution to extract the cellular ATP. Thetubes were vortexed and allowed to incubate for 10 minutes at roomtemperature.

Using disposable plastic pipet tips, the TCA extracts were transferredto luminometer cuvettes (Sarstedt) and neutralized with 400 μl of a 0.1MTris-acetate buffer, pH 7.75. To initiate the luciferase reaction, 100μl of a P. pyralis luciferase-luciferin reagent (Promega Corp., Madison,Wis., USA) was added, and the light output measured using a Berthold9501 luminometer (Berthold Analytical, Munich, Germany) using a 10second integration time. Light output results are printed out asrelative light units (RLU).

Using equivalently prepared contaminated milk samples (Tubes 1-11 listedin Table 1 above), 50 μl samples of each milk sample was analyzed usinga commercially available milk bacterial detection kit (Lumac® by, TheNetherlands) using the manufacturer's protocol.

Results from these assays are summarized in FIG. 2. Log of RLU (y-axis)is plotted versus log of bacterial number (x-axis) in colony formingunits (CFU) per milliliter for the present invention and thecommercially available milk assay kit. Sensitivity for the presentinvention was found to be less than 1×10⁴ cells/ml while the sensitivityfor the Lumac assay was approximately 1×10⁶ cells/ml.

EXAMPLE 2

This example discloses the separation of microbial cells from eleven rawmilk samples and one pasteurized milk sample.

Raw milk samples were received from a local dairy and stored at 4° C. Apasteurized milk sample was also included in the study. One ml of eachmilk sample was pipetted in duplicate into 1.5 ml Eppendorf tubes. Themilk samples were allowed to incubate at room temperature for 10 minutesfollowed by the addition of 0.5 ml of 0.5% NP-40, 3% nitrilotriaceticacid (sodium salt) to each tube. The tubes were then capped and mixed byinverting 10 times.

The tubes were centrifuged at room temperature for 5 minutes at 12,000×g and the supernatants aspirated as in Example 1. To each pellet 0.5 mlof a 0.05 mM MgCl₂, 0.2% NP-40 solution was added, the tubes were cappedand vortexed, and centrifuged for 5 minutes as before. Aftercentrifugation the supernatants were again aspirated, and the washing,centrifugation and aspiration were repeated one more time.

The pellets obtained were treated with TCA solution and read in aluminometer, as described above in Example 1. For each sample, theresult is determined as the mean of the duplicate measurements.

Each milk sample was also diluted with sterile 0.8% NaCl and 1.0 ml of10-fold dilutions were pipetted into duplicate petri dishes.Approximately 20 ml of sterile standard methods agar was added to eachdish and mixed. The plates were incubated at 23° C. for 48 hours andcolonies were then counted. Results are the means of duplicate platecounts.

The results of this example are shown in FIG. 3. A linear responsebetween log colony forming units (CFU)/ml and log RLU is obtained overthe range studied. The correlation coefficient for this data was 0.994,indicating a significant relationship between the two methods.

EXAMPLE 3

This example discloses the separation of microbial cells from 65 rawmilk samples using a modification of the procedure presented in Example2.

The milk samples were treated as in Example 2, with an additional stepof adding a protease treatment to treat the microbial pellet.

After the first centrifugation step the resulting pellet was dissolvedin 500 μl 0.05 mM MgCl₂, 0.2% NP-40, and 30 μg/ml α-chymotrypsin (SigmaChemical Co., St. Louis, Mo., USA, Cat. No. C7762). The pellet wasvortexed three times for 2 seconds and the tubes were incubated at roomtemperature for 20 minutes. The remainder of the procedure is identicalto that described in Example 2.

FIG. 4 shows the correlation of log RLU to log CFU/ml for the 65 rawmilk samples using this procedure. It indicates a positive correlationbetween the two methods.

EXAMPLE 4

This example discloses the separation of microbial cells from 88 rawmilk samples using a modification of the procedure presented in Example3.

To 1.0 ml of each raw milk sample was added 500 μl of a solution of 3%nitrilotriacetic acid (sodium salt), 0.5% Triton X-100, and the sampleswere then treated as in Example 3. After aspirating the supernatantsfrom the first centrifugation step, a solution of 0.05 mM MgCl₂, 0.2%Triton X-100 containing 0.01% surfactant-free polystyrene beads (0.984μm, Bangs Labs, Carmel, Ind., USA) and 60 μg/ml α-chymotrypsin. Thepolystyrene beads centrifuge down with the cells. The samples wereincubated, centrifuged, and aspirated as in Example 3, and the pelletswere resuspended in 0.05 mM MgCl₂, 0.2% Triton X-100, vortexed, andrecentrifuged. The remainder of the procedure was performed as describedin Example 3. The results of this experiment are shown in FIG. 5. Apositive correlation between the two methods was obtained in this study.

EXAMPLE 5

This example discloses the separation of microbial cells from 18 rawmilk samples using a modification of the procedure described in Example4.

In this study an equivalent amount of carrier polystyrene beads(described in Example 4) were added to the 3% nitrilotriacetic acid(sodium salt), 0.5% Triton X-100 solution for the first milk treatmentstep. Carrier was not added in any subsequent steps. In addition,α-chymotrypsin was used as a final concentration of 150 μg/ml in thefirst wash solution. The remainder of the procedure is as described inExample 4.

The results of this correlation study are shown in FIG. 6, which shows acorrelation between the two procedures.

EXAMPLE 6

This example demonstrates the usefulness of the present invention usinga filtration device in the procedure.

A set of inoculated pasteurized milk samples was prepared as inExample 1. Samples corresponding to tubes 1 through 11 in Table 1(Example 1) were used.

To each sample was added 300 μl of 0.5M EDTA, pH 8.0, and 300 μl of 1%NP-40. The tubes were capped, inverted 10 times to mix, and centrifugedat 12,000× g for 10 minutes. The supernatants were removed by aspirationand the pellets resuspended in 200 μl of 0.1M MgCl₂, 0.2% NP-40 andvortexed to resuspend the pellets. These cell suspensions were put intoa spin filter device (Millipore, Ultrafree MC, 0.45 μm) and centrifugedat 12,000× g for 10 minutes. After centrifugation the filter insert wasremoved to a capless sterile microcentrifuge tube, and 50 μl of 1% TCAwas added to the filter and allowed to incubate for 10 minutes at roomtemperature. The filters and holders were re-centrifuged for 10 minutesand the TCA extracts were collected and analyzed as in Example 1.

The results of this experiment are shown in FIG. 7 and demonstrate theusefulness of the present invention in a filtration-type format. A gooddose response of RLU is shown over the cell concentration range studiedin this example.

EXAMPLE 7

Salmonella typhimurium strain PB637, obtained from a hospital in NewEngland, was grown overnight in a rich broth. The overnight culture wasdiluted to an OD₆₀₀ of 0.1 and then grown to an OD₆₀₀ of 0.9. Theculture was serially ten-fold diluted in phosphate-buffered saline(PBS). An aliquot of each dilution was plated to determine the titer ofviable bacteria. Five μl of each dilution was added to 0.995 ml of rawmilk in microcentrifuge tubes. There were 7 dilutions plus one blank ofPBS with no bacteria. Next 500 μl of the clearing solution (0.25M EDTA,0.5% Triton X-100, 0.01% microparticulate carrier (surfactant-freepolystyrene beads, see Example 4)) was added to each tube. The tubeswere inverted ten times to mix thoroughly. The tubes were centrifugedfor 5 minutes in a microcentrifuge. The cream layer was removed and thesupernatant was aspirated. The cells of the pellet were then washed byresuspending the pellet in 1 ml of PBS with the use of a vortex mixer,followed by adding 500 μl of the resulting cell suspension to amicrocentrifuge tube and centrifuging for 5 minutes. The supernatant wasremoved by aspiration and the resulting pellet was resuspended in 25 μlof distilled water.

Amplification of a 324 base-pair segment of the Salmonella genome (inthe ompA gene, for sequence see Freudl and Cole, Eur. J. Biochem.134:497-502 (1983)) was used for detection of the Salmonella present inthe raw milk sample. Oligonucleotides designated A86, a 28-base DNA ofsequence 5'-GTAGACGGGC CGCCAGGGAC GTTAGACT(SEQ ID NO:1), and B83, also a28-base DNA, of sequence 5'-ACGCTGGTGC TAAACTGGGC TGGTCTCA(SEQ ID NO:2),were used as the primers. The primers were mixed such that each primerwas at a concentration of 50 μM in sterile, distilled water.

A PCR reagent mix was made by mixing 180 μl of 10× Taq DNA PolymeraseBuffer (500 mM KCl, 100 mM Tris-HCl (pH 8,8 at 25° C.), 15 mM MgCl₂, 1%Triton X-100)(Promega Corp., Madison, Wis., USA), 10.8 μl (45 units) ofTaq DNA Polymerase (Promega Corp.), 180 μl of dNTP solution (2 mM ofeach of dATP, dGTP, dCTP and TTP) to provide an initial concentration of200 μM of each of the dNTP's in the PCR reaction, plus distilled waterto a final volume of 1.8 ml. To a new microfuge tube was added 93 μl ofthe PCR reagent mix and 2 μl of the primer mix (100 pmoles of eachprimer). Next 5 μl of each bacterial suspension was added to the tubeand the liquid was overlaid with 2 drops of mineral oil. The tubes wereplaced in a Perkin-Elmer Cetus DNA Thermal Cycler (Perkin-Elmer Cetus,Norwalk, Conn., USA). There were no steps taken to lyse the bacteria orextract the DNA. This was accomplished during the PCR reactions. TheThermal Cycler conditions were set for 1 min at 94° C., then 1 min at65° C., and finally 2.5 minutes at 72° C. with 5 second autoextend. Atotal of 35 cycles of PCR were performed.

The PCR products were analyzed by agarose gel electrophoresis. A 1.5%agarose gel in TAE buffer was loaded with 5 μl of each PCR reactionproduct. The gel was electrophoresed for 1.5 hr at 70 V. The gel wasstained with ethidium bromide. The lane from the milk sample with 250cells/ml of raw milk showed a visible band of the expected fragmentlength. Dilutions with fewer cells and the PBS control tube (PBS with nocells added to milk) showed no bands on the gel.

The gel was denatured in 0.5M NaOH for 30 minutes at room temperature.It was then washed two times in 1M Tris-HCl for 15 minutes each wash.The gel was then blotted by overlaying the gel with a nylon membranefollowed by 0.5 inches of Whatman 3MM paper and Saran wrap, weightedwith two large books. After 2 hours, the nylon was washed for 10 minutesin 2× SSC and UV crosslinked in a UV-Stratalinker 1800 (Stratagene, LaJolla, Calif., USA) in automatic mode. The gel was hybridized with a ³²p kinase-labelled, 24-base DNA, designated P47, overnight at 62° C. in2× SSC, 20 mM sodium phosphate, 0.1% SDS, 10× Denhardt's solution, 10%dextran sulfate, and 0.1 mg/ml herring sperm DNA. The nylon was washed 2times for 15 minutes each at 62° C. in 2× SSC with 0.1% SDS. The nylonwas exposed to Kodak XAR film at -70° C. for 5.5 hours. Probe P47 isSalmonella-specific and has the sequence of a segment of the SalmonellaompA gene between the segments with the sequences of the two primersused in the amplification. The sequence of P47 is 5'-ATAAGCGCCATTGATGTTGT CGCC(SEQ ID NO:3). All of the dilutions of bacteria into rawmilk produced a PCR product band that hybridized with P47 while the PBScontrol did not. The highest dilution had approximately 2.5 viable cellsadded to the 1 ml of raw milk. Only 1/10th of this material was added tothe PCR reaction, indicating that either the sample taken for the PCRreaction just happened to contain one cell or that the culture had somenon-viable cells present. In either case, it is clear that the procedureis highly sensitive for detecting Salmonella cells in raw milk.

EXAMPLE 8

An experiment was done to detect Salmonella typhimurium in beef steak.Five samples (25 g) of beef steak were each added to 225 mltetrathionate broth and stomached in a Stomacher Lab-Blender 400 (TekmarCo., Cincinnati, Ohio, USA) with 0, 0.02, 2, 200, and 20,000 Salmonellacells/ml as determined by an initial standard plate count (SPC) of theinoculum done on bismuth sulfite agar plates. The samples were incubatedat 37° C. with shaking (150 rpms). At 0, 3 and 24 hours, a plate countand "clearing wash PCR assay" (an assay similar to that described inExample 7, beginning with the clear wash procedure to obtain the initialcell pellet) were performed on each of the five broths. The plate countswere also done on bismuth sulfite agar (selective for and indicative ofthe growth of the Salmonellae), since it was anticipated that the meatwas precontaminated with bacterial flora. Table 8 provides results fromthe counts on bismuth sulfite agar.

                  TABLE 8                                                         ______________________________________                                        Bismuth Sulfite Plate Counts in Cells/ml                                      # Salmonella Added per Milliliter                                             0           0.02     2        200    20,000                                   ______________________________________                                        T = Oh 0        0        1.2 × 10.sup.2                                                                 7.0 × 10.sup.2                                                                 3.5 × 10.sup.4                   T = 3h 0        0        1.0 × 10.sup.2                                                                 4.5 × 10.sup.3                                                                 3.6 × 10.sup.5                   T-24h  9.5 × 10.sup.6                                                                   1.2 × 10.sup.9                                                                   1.6 × 10.sup.9                                                                 1.8 × 10.sup.9                                                                 1.6 × 10.sup.9                   ______________________________________                                    

The clearing solution was as in Example 7 (0.25M EDTA pH 8.0/0.5% TritonX-100/0.01% carrier). The pellets were resuspended in 100μl PBS and thenquickly frozen at -70° C. After all the samples had been taken andprepared, they were thawed and vortexed. 5 μl were withdrawn from eachand added to 95 μl of PCR reaction mix (see Example 7). PCRamplification was carried out using the Perkin-Elmer Cetus DNA ThermalCycler with oligonucleotides designated C84 (a 27-base DNA of sequence5'-CGGCTTCATT CACAATGATG GCCCGAC)(SEQ ID NO:4) and A86 as primers. Theprimers bracket a 284 base-pair segment of the Salmonella ompA gene;this 284-base-pair segment includes a subsegment with the sequence ofprobe P47. The PCR cycle consisted of 1 min at 95°, 1 min at 65°, and 2min 30 sec at 72° with a 5 sec autoextend. A 1.5% agarose gel was run onthe PCR products. The gel was then treated with NaOH, neutralized, andsquash-blotted onto a nylon membrane. The membrane was rinsed and UVcrosslinked before being probed with ³² P kinase-labelled P47 as probe.Subsequent autoradiography confirmed that the products were amplifiedSalmonella genomic DNA segments which included a subsegment with thesequence of P47. A positive signal was noted for the 20,000 cells per mlsamples at 0 and 3 hours incubation and at all inoculationconcentrations at 24 hours incubation time. Thus, the clearing washmethod to concentrate cells following an overnight culture prepared frommeat samples is a satisfactory method of sample preparation for PCRanalysis for microorganism contamination.

EXAMPLE 9

An experiment was performed to demonstrate the significant improvementin ATP recovery from microbial cells recovered from raw milk samplesusing a modification of the procedure described in Example 5.

In this study cells were isolated as in Example 5 and 0.05 ml of asolution containing 50 μg/ml α-chymotrypsin and 0.001 U/ml potatoapyrase (Sigma Chemical, St. Louis, Mo., USA, Grade VI) was added to themicrobial pellet. The pellet was mixed with the solution, incubated forapproximately 5 minutes at room temperature, and transferred to aluminometer cuvette. A solution containing 0.075% (v/v) chlorhexidinedigluconate and 0.00075M Trilon (0.10 ml) was added to the pelletsuspension and 0.1 ml of a luciferase/luciferin solution was added tothe cuvette. Light output was measured over a 10 second interval andreported as RLU. The results of this procedure are shown in FIG. 8 whichshows a correlation between this procedure and standard plate counting.

EXAMPLE 10

An experiment was performed to demonstrate the significant improvementof nitrilotriacetic acid (Trilon) over EDTA in an ATP-basedbioluminescent estimation of milk microbiological load. A milk isolatedorganism (Serratia liquefaciens) was inoculated into Luria broth andshaken overnight at 37°. Cells from the overnight culture were spikedinto a pasteurized milk sample to give a final bacterial concentrationof 4.7×10⁷ colony forming units per ml of milk. This milk sample wasfurther diluted with pasteurized milk to cover a bacteria range from4.7×10⁷ cfu/ml to 9.5×10³ cfu/ml. One ml aliquots of each milk samplewere mixed with 0.5 ml of one of three types of chelating agentsolutions. The first type of solution (chelator 1) was 0.13Mnitrilotriacetic acid pH 6.8, 0.5% Triton X-100, and 0.01% microparticlecarrier (0.787 micron diameter); the second type of solution (chelator2) was 0.25M EDTA pH 8, 0.5% Triton X-100, 0.01% microparticle carrier(0.787 micron), and the third type of solution (chelator 3) was 0.25MEDTA pH 12.0, 0.5% Triton X-100, 0.01% microparticle carrier (0.787micron). Milk samples containing chelating solutions were capped, mixedby inversion and centrifuged for 5 minutes at 12000× g. Supernatantswere aspirated from the bacterial microparticle carrier pellets and 100microliters of a solution containing 0.2% Triton X-100, 0.05 mMmagnesium chloride, 0.5% penicillin/streptomycin, 0.05 units/ml ofpotato apyrase (an ATPase)(Sigma, Grade VI) and 50 μg/ml α-chymotrypsinwas added to each pellet. The pellet was resuspended and the entirecontents of the microcentrifuge tube was transferred to a luminometercuvette. The cuvette was placed into a Model 953 Berthold luminometerand 100 microliters of a solution containing 0.075% chlorhexidinedigluconate, 0.015% CTAB (Cetyltrimethylammonium bromide) and 0.0015MTrilon. After a one second delay, 100 microliters of aluciferase-luciferin ("L/L") solution was added and light output wasmeasured over a ten second integration time. Results from thisexperiment are shown in FIG. 9. Each curve represents a dilution seriesof milk samples containing variable amounts of Serratia liquifaciens.The first curve in the open squares is a Trilon chelating curve anddemonstrates a very nice dose response over three orders of magnitude ofcfu/ml. The second and third curves that demonstrate the chelating andbackground reduction ability of EDTA are also shown in the opentriangles and open diamonds. Both curves demonstrate adequate doseresponse at cfu levels greater that 1×10⁵ although there is a reductionin the RLU recovery from samples that have been chelator treated withEDTA at pH 12. The significant point in this figure is the backgroundthat remains at cell titers less than 1×10⁵. Both of the EDTA curvesfail to detect cells at an RLU level less than 1×10⁵. This effectivelywould reduce the potential sensitivity of an assay to about 5×10⁵ colonyforming units if an EDTA type solution were used. Utilizing a Trilonbased chelating solution however, allows an investigator to detect10-fold less cells, down to 10⁴ cfu/ml, thus giving a better assaysystem.

EXAMPLE 11

It was found that N-(2-acetamido) iminodiacetic acid (ADA) is equivalentto nitrilotriacetic acid as a chelator in analyses based ondeterminations of ATP by bioluminescence. A raw milk sample was seriallydiluted with a pasteurized milk sample to yield a series of samples withvarious amounts of raw milk present. The samples were divided into twosets and one set was chelator treated with a solution containing 0.13 Mnitrilotriacetic acid, 0.5% Triton X-100, and 0.01% microparticlecarrier, as in Example 10. The second set of milk samples were treatedwith a solution containing 0.25M (N-(2-acetamido) iminodiacetic acid)(ADA), 0.5% Triton X-100, 0.01% microparticle carrier, also as inExample 10. Both sets of spiked milk samples were then assayed asdescribed in Example 10. The results of the experiment showed that ADAis an equivalent chelating agent to nitrilotriacetic acid for this typeof application.

It is understood that the invention is not limited to the particularembodiments described herein, but embraces such modified forms thereofas come within the scope of the following claims.

    __________________________________________________________________________    SEQUENCE LISTING                                                              (1) GENERAL INFORMATION:                                                      (iii) NUMBER OF SEQUENCES: 4                                                  (2) INFORMATION FOR SEQ ID NO:1:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 28 bases                                                          (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: single                                                      (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: genomic DNA                                               (iii) HYPOTHETICAL: no                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                       GTAGACGGGCCGCCAGGGACGTTAGACT28                                                (2) INFORMATION FOR SEQ ID NO:2:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 28 bases                                                          (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: single                                                      (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: genomic DNA                                               (iii) HYPOTHETICAL: no                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                       ACGCTGGTGCTAAACTGGGCTGGTCTCA28                                                (2) INFORMATION FOR SEQ ID NO:3:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 24 bases                                                          (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: single                                                      (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: genomic DNA                                               (iii) HYPOTHETICAL: no                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                       ATAAGCGCCATTGATGTTGTCGCC24                                                    (2) INFORMATION FOR SEQ ID NO:4:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 27 bases                                                          (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: single                                                      (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: genomic DNA                                               (iii) HYPOTHETICAL: no                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                       CGGCTTCATTCACAATGATGGCCCGAC27                                                 __________________________________________________________________________

We claim:
 1. A method for detecting the presence of cells having nucleicacid that comprises a preselected target segment, in a culture orextract of a food material, said method comprising obtaining a pellet ofcells from the culture or extract by combining an aliquot of the cultureor extract with an aqueous suspension of a microparticulate carrier toform a clearing solution and centrifuging the clearing solution to forma cell pellet, provided that, if said culture or extract of a foodmaterial is a culture and is a liquid milk sample, said clearingsolution further comprises a chelating agent; suspending cells from thepellet so obtained in a first solution and treating the first solutionto provide a second solution of nucleic acid from the cells foramplification without substantial isolation of nucleic acids of thecells from other constituents of the cells; subjecting the nucleic acidof the second solution to a nucleic acid amplification process toproduce a pre-determined, amplified nucleic acid segment only if thepre-selected target segment is present in said cells suspended from saidpellet; and assaying nucleic acid after the amplification process forthe presence of said pre-determined, amplified segment.
 2. The method ofclaim 1 wherein said food material is milk, and the culture or extractof said material is a culture and is a liquid milk sample.
 3. The methodof claim 2 wherein the chelating agent is EDTA.
 4. The method of claim 1wherein the food material is other than milk.
 5. The method of claim 4wherein said food material is a meat.
 6. The method of any one of claims1-5 wherein the microparticulate carrier is polystyrene beads with adiameter of about 0.5 μm to 1.5 μm.
 7. The method of any of claims 1-5wherein the amplification process is a PCR process.
 8. The method ofclaim 7 wherein the microparticulate carrier is polystyrene beads with adiameter of about 0.5 μm to 1.5 μm.
 9. The method of claim 7 wherein aTaq DNA polymerase is used to catalyze DNA synthesis in the PCRamplification process.
 10. The method of claim 8 wherein a Taq DNApolymerase is used to catalyze DNA synthesis in the PCR amplificationprocess.
 11. The method of claims 9 wherein the cells to be detected areof Salmonella spp.
 12. The method of claim 10 wherein the cells to bedetected are of Salmonella spp.
 13. A test kit for the detection ofcells having nucleic acid that comprises a preselected target segment,in a culture or extract of a food material, said kit comprising amicroparticulate carrier and, if said culture or extract of a foodmaterial is a culture and is a liquid milk sample, a chelating agent;enzyme and primers or probes necessary for a nucleic acid amplificationprocess to produce a pre-determined, amplified nucleic acid segment onlyif the preselected target segment is present in cells of said culture orextract; and reagents required for assay of nucleic acid after theamplification process for the presence of said pre-determined, amplifiedsegment.
 14. The kit of claim 13 for detection of cells in liquid milksamples.
 15. The kit of claim 14 wherein the chelating agent is EDTA.16. The kit of claim 13 for detection of cells in a culture or extractof a food material other than milk.
 17. The kit of claim 16 wherein saidfood material is a meat.
 18. The kit of any of claims 13-17 wherein themicroparticulate carrier is polystyrene beads with a diameter of about0.5 μm to 1.5 μm.
 19. The kit of any of claims 13-17 wherein theamplification process to be carried out is a PCR process.
 20. The kit ofclaim 19 wherein the microparticulate carrier is polystyrene beads witha diameter of about 0.5 μm to 1.5 μm.
 21. The kit of claim 19 furthercomprising a Taq DNA polymerase for use in catalyzing DNA synthesis inthe PCR amplification process.
 22. The kit of claim 20 furthercomprising a Taq DNA polymerase for use in catalyzing DNA synthesis inthe PCR amplification process.
 23. The kit of claim 21 wherein the cellsto be detected are of Salmonella spp.
 24. The kit of claim 22 whereinthe cells to be detected are of Salmonella spp.